Tuning fuel composition for driving cycle conditions in spark ignition engines

ABSTRACT

Tuning fuel composition delivered to a spark ignition, internal combustion engine as a function of driving cycle conditions results in improvements in one or more of fuel efficiency and combustion emissions.

FIELD OF INVENTION

This is a divisional application that claims priority to, andincorporates by reference, U.S. Ser. No. 09/818,210 filed Mar. 27, 2001.The present invention relates generally to engine fuel compositions andtheir use in port or direct fuel injection spark ignition, internalcombustion engines especially those having a compression ratio (CR) of11 or more.

BACKGROUND OF INVENTION

Both petroleum refineries and engine manufacturers are constantly facedwith the challenge of continually improving their products to meetincreasingly severe governmental efficiency and emission requirements,and consumers' desires for enhanced performance. For example, inproducing a fuel suitable for use in an internal combustion engine,petroleum producers blend a plurality of hydrocarbon containing streamsto produce a product that will meet governmental combustion emissionregulations and the engine manufacturers performance fuel criteria, suchas research octane number (RON). Similarly, engine manufacturersconventionally design spark ignition type internal combustion enginesaround the properties of the fuel. For example, engine manufacturersendeavor to inhibit to the maximum extent possible the phenomenon ofauto-ignition which typically results in knocking and, potentiallyengine damage, when a fuel with insufficient knock-resistance iscombusted in the engine.

Under typical driving situations, engines operate under a wide range ofconditions depending on many factors including ambient conditions (airtemperature, humidity, etc.), vehicle load, speed, rate of acceleration,and the like. Engine manufacturers and fuel blenders have to designproducts which perform well under such diverse conditions. Thisnaturally requires compromise, as often times fuel properties or engineparameters that are desirable under certain speed/load conditions provedetrimental to overall performance at other speed/load conditions.

One object of this invention to provide an engine with fuelsspecifically designed to enhance engine performance at low and high loadengine conditions.

Another object of the invention is to provide an engine with fuelsspecifically designed to enhance engine performance across the drivingcycle.

Also, spark ignition engines are generally designed to operate at acompression ratio (CR) of 10:1 or lower to prevent knocking at highload. As is known, higher CRs, up to about 18:1, are optimum from thestandpoint of maximizing the engine thermal efficiency across the loadrange. Compression Ratio (CR) is defined as the volume of the cylinderand combustion chamber when the piston is at Bottom Dead Center (BDC)divided by the volume when the piston is at Top Dead Center (TDC). Ahigher CR leads to greater thermal efficiency by maximizing the workobtainable from the theoretical Otto (engine compression/expansion)cycle. Higher CRs also lead to increased burn rates, giving a furtherimprovement in thermal efficiency by creating a closer approach to thisideal Otto cycle. The use of high compression ratio spark ignitionengines, however, is limited by insufficiently high fuel octane, as inpractice it is difficult to supply a single fuel with sufficiently highoctane overall to allow for a significant increase in compression ratiowithout having engine knocking at high loads.

Therefore, another objective of this invention is to facilitate thedesign of high compression ratio engines that realize greater thermalefficiency across the entire driving cycle without the problem ofknocking at high load.

In theory, higher efficiency engine operation at certain moderate tohigh loads can be achieved by adjusting the spark ignition timing closerto the value that provides MBT spark timing. MBT is defined as minimumspark advance for best torque. Experience has shown, however, thatadjusting the ignition timing to allow MBT to be reached is notpractical since knocking typically occurs under conditions of moderateto high load at timings earlier than MBT with commercially availablegasolines. In principle, operating with a very high octane fuel wouldallow running the engine at MBT across the drive cycle. We will showbelow that a more preferred approach is to supply the engine with a fuelthat has sufficient octane to approach or operate at MBT withoutknocking, together with other combustion properties tailored to optimizeperformance.

Yet another object of the invention is to provide fuel compositions thatallow adjusting the spark ignition timing closer to that which providesMBT.

Presently spark ignition engines are capable of operating with knownfuels at a normalized fuel to air ratio (“φ”) below 1.0 under low tomoderate load conditions. The normalized fuel to air ratio is the actualfuel to air ratio divided by the stoichiometric fuel to air ratio. Inaddition, these engines can be operated with exhaust gas recycle (EGR)as the “leaning out” diluent, at a φ of 1.0 or lower. EGR is understoodto include both recycled exhaust gases as well as residual combustiongases. One challenge associated with operating the engine lean is thedifficulty of establishing a rapid and complete burn of the fuel.

Another object of this invention therefore is to provide high burn ratefuel for use under lean conditions to shorten the burn duration andthereby improve the thermodynamic efficiency. A faster burn rate alsoserves to maximize conversion of the fuel, thereby increasing theoverall fuel economy and reducing emissions. As known in the art,autoignition of the fuel at sufficiently high loads can pose a threat ofmechanical damage to the engine, i.e., knocking. However, at certain lowload conditions, for example lean stratified operation, autoignition ofthe fuel can be beneficial to overall engine operation by optimizingburn characteristics that result in reduced engine emissions and higherefficiency. An additional object of this invention, therefore is toprovide a high autoignition tendency, low octane fuel. A further objectis to provide a high laminar flame speed fuel.

Other objects of the invention and their attendant advantages will beapparent from the reading of this specification.

SUMMARY OF INVENTION

One aspect of the invention is the provision of a plurality of unleadedfuel compositions for use in operating a spark ignition, internalcombustion engine, especially an engine having a CR of 11 or more, eachof which compositions have different predetermined combustion propertiessuitable for use under preselected engine operating conditions toimprove one or more of fuel efficiency and combustion emissions.

In one embodiment at least a first and second fuel composition isprovided, the first fuel having combustion properties sufficient toimprove combustion thereof under high engine load conditions and thesecond fuel having combustion properties sufficient to improvecombustion thereof under low engine load conditions.

Especially preferred fuels for use under low load conditions are thoseunleaded fuels boiling in the gasoline boiling range that have a RONless than 90 and an average burn rate in the engine, defined as 1/ crankangles for 90% burn completion, >105% % of isooctane at this time in theengine operating cycle and a laminar flame speed >105% % of isooctanemeasured at a temperature and pressure representative of conditions inthe engine at or about this time in the engine operating cycle.

Especially preferred fuels for use under low load conditions are thoseunleaded fuels boiling in the gasoline boiling range that have a RONless than 90 and an average burn rate in the engine defined as 1/ crankangles for 90% burn completion, >105% % of isooctane at this time in thecycle and a laminar flame speed >105% % of isooctane measured at atemperature and pressure representative of conditions in the engine atthe low end of the load scale.

In view of the foregoing it will be readily appreciated that a widerange of modifications and variations of the invention are within thebroad aspects set forth above and the unique scope of the invention willbecome even more apparent upon a reading or the detailed descriptionwhich follows.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 demonstrates the effect of fuel composition and compression ratioon output torque for a fuel of the invention compared to isooctane.

FIG. 2 compares relative engine brake efficiency vs. spark advance forisooctane and one fuel of the invention.

FIG. 3 are graphs of Burn Rate and Heat Release Rate that compare burncurves for isooctane and one fuel of the invention.

FIG. 4 demonstrates the effect on output torque of fuel composition andcompression ratio at various injection timings for a fuel of theinvention compared to reference fuel LFG-2B

FIG. 5 demonstrates emission benefits obtained by the invention.

FIG. 6 demonstrates the effect of higher compression ratio and fuelcomposition on output torque for a fuel of the invention compared toreference fuel LFG-2B.

FIG. 7 compares burn curves for reference fuel LFG-2B and one fuel ofthe invention.

FIG. 8 demonstrates emission benefits obtained by the invention.

FIG. 9 compares engine brake efficiency at constant NOx emissions forreference fuel LFG-2B and one fuel of the invention.

FIG. 10 compares emissions data for one fuel of this invention referencefuels LFG-2B and RON91 at a medium load condition.

FIG. 11 demonstrates the effect of fuel composition and compressionratio on relative output torque for a fuel of the invention compared toreference fuels LFG-2B and RON91

FIG. 12 demonstrates relative fuel efficiency improvements obtained bythe invention.

FIG. 13 demonstrates fuel efficiency and emission improvements obtainedby the invention.

[FIG. 4 compares heat release curves for reference fuel isooctane andseveral low octane test fuels.

DETAILED DESCRIPTION OF INVENTION

As is well known in the art, gasoline fuels generally are composed of amixture of hydrocarbons boiling at atmospheric pressure in the range ofabout 77° F. (25° C.) to about 437° F. (225° C.). Typically gasolinefuels comprise a major amount of a mixture of paraffins, cycloparaffins,olefins and aromatics, and lesser, or minor amounts of additivesincluding oxygenates, detergents, dyes, corrosion inhibitors and thelike. Typically also, gasoline fuels are formulated to have a RON ofabout 98 for premium grade and about 92 for regular grade and are usedalone in a vehicle engine the grade used normally depending upon thevehicle manufacturer's recommendation.

The present invention departs form the practice of formulating a singlefuel for a specific vehicle engine. Indeed, the present invention isbased on the discovery that significant benefits can be achieved byproviding a range of fuel compositions having combustion propertiestailored to the engine's specific operating condition.

The fuel compositions of the invention are unleaded fuels boiling in thegasoline range and capable of being used in spark ignition, internalcombustion engines especially those having a CR of 11 or higher.

In one embodiment the fuel compositions will comprise at least one firstfuel and a second fuel. The first fuel will have a RON greater than 100,and a burn rate greater than 105% of isooctane at the high load end ofthe cycle and a laminar flame speed of greater than 105% of isooctanemeasured at a temperature and pressure representative of conditions inthe engine at the high end of the load scale. The second fuel will havea RON less than 90, a burn rate greater than 105% of isooctane at thelow end of the cycle and a laminar flame speed greater than 105% ofisooctane measured at a temperature and pressure representative ofconditions in the engine at the low end of the load scale.

The laminar flame speed of the fuel compositions is measured bycombustion-bomb techniques that are well known in the art. See, forexample, M. Metghalchi and J. C. Keck, Combustion and Flame, 38:143-154(1980).

A particularly useful unleaded fuel for operating the engine in the highload portion of the drive cycle comprises a mixture of hydrocarbonsboiling in the gasoline range with an RON>100 and containing greaterthan about 45 vol % aromatics and preferably greater than about 55 vol%.

A particularly useful unleaded second fuel for operating the engine inthe low load portion of the drive cycle comprises a mixture ofhydrocarbons boiling in the gasoline range having an RON<90 andcontaining less aromatics than the first fuel, for example, less thanabout 45 vol % aromatics and preferably less than 20 vol %.

Fuels meeting the foregoing characteristics provide efficiency benefitsfor various types of spark ignited internal combustion engines whenoperating under high load conditions. High load conditions are definedas being those regions of the engine operating map where at MBT sparktiming knocking occurs with a gasoline of RON 98. Knocking is defined asautoignition under sufficiently severe in-cylinder conditions that itresults in a detonation that poses a risk of mechanical damage to theengine.

In the case of port fuel injection engines, use of fuels having theproperties of the first fuel above permits the engine to be designed tooperate at a CR of 11 or more and permits advance spark timing closer tothat for MBT. These design features enhance overall cycle efficiency,i.e., provide improved fuel economy.

More particularly these benefits are achieved with direct fuel injectionengines and especially direct injection, lean burn engine systems, suchas stratified charge direct injection systems. Stratified charge is anin-cylinder condition wherein there is an inhomogeneous air:fuel ratiodistribution. As is known, “lean burn” engines operate at normalizedfuel to air ratios (“φ”) of below 1.0 and/or with exhaust gas recycle asthe “leaning out” diluent, at a φ of 1.0 or lower.

Fuels having the combustion properties of the second fuel above aresuitable for use especially in the operation of spark ignition engines,included stratified fuel systems, operating under low load conditionswith exhaust gas recycle. Low engine load conditions are those regionsof the engine operating map at or below which the engine can be operatedat MBT timing with a fuel having a RON of approximately 90 without thecondition of knocking as defined above.

Fuels having a range of combustion properties between the first andsecond fuel offer even more complete tuning of the fuel compositions toengine operating conditions. Indeed, a third fuel composition can beprovided having a RON between those of the first and second fuel, andmost desirably a burn rate greater than 105% of isooctane at the mediumload end of the cycle and most desirably a laminar flame speed ofgreater than 105% of isooctane measured at a temperature and pressurerepresentative of conditions in the engine at the medium end of the loadscale. Such a fuel can be used under moderate engine load conditions,i.e., conditions wherein the octane required for MBT is less than 98 RONand more than 90 RON.

One way to achieve the benefits of the invention is by supplying thehigh octane fuel to an engine at the high end of the engine load scale,for example, and the low octane fuel at the low end of the engine loadscale. There are many ways in which this can be accomplished. Forexample, two fuel tanks, one containing the first and the othercontaining the second fuel can be provided with the fuel supplied to theengine being based on a predetermined engine condition. The electroniccontrol unit map will be the basis for this decision. Optionally, thefirst and second fuels can be blended in appropriate proportions toprovide a third fuel to be supplied to the engine under moderate loadconditions.

In yet another embodiment of the invention a single fuel, i.e., aregular grade gasoline of about 92 RON is stored in a vehicle primaryfuel tank. Under moderate engine load conditions fuel is supplieddirectly to the engine. A portion of the fuel from the primary tank isalso separated into two streams. Under high load conditions a first fuelstream having a RON greater than 100 and greater than 45 vol. %aromatics which is stored for use at high load conditions, is deliveredto the engine. Under low load conditions, a second fuel stream of RONless than 90 and less aromatics than the first fuel which is stored in asecondary tank is supplied to the engine. Separation of the fuel intothe two streams is achieved preferably by pervaporation membranesseparation techniques (See for example patent EP466469 which teaches useof a polyethylene terephtalate membrane for separation of gasolineboiling range aromatics and nonaromatics under pervaporation conditions,which is incorporated herein by reference.)

In another embodiment the invention is applicable to engines thatoperate under high exhaust gas recycle, i.e., 20% or greater, during thelow to moderate engine load.

EXAMPLES Example 1

The effects of a high octane, high knock-resistant, high burn rate fuelon combustion efficiency and performance were investigated in an in-line4-cylinder (2.0 L displacement) DOHC 4 valve/cylinder direct injectionspark ignition engine with a shell-shaped piston cavity, a straightintake air port, and a fan-shaped fuel spray. The engine was operated athigh load/wide open throttle (WOT) at a compression ratio of 13.0. Thebase fuel was pure iso-octane with RON=100. The test fuel, called “DF-2”was comprised of 60% toluene, 33% iso-octane, and 7% n-heptane (measuredRON=103). The fuel properties are listed in Table 1. Both fuels werecombusted under the following conditions: engine speed=4000 rpm,fuel/air ratio (φ)=1.15, spark advance timing=11-24 degrees before topdead center (BTDC). In this example and the others that follow, theinjection quantities of the fuel are adjusted so as to maintainequivalent total heating values TABLE 1 FUEL PROPERTIES FOR WOT TESTSTest Fuel DF-2 Isooctane Density g/cm³ @ 15° C. 0.7945 0.694 RON — 103.1100 MON — 93.2 100 LHV KJ/g 44.4 H/C mol/mol 1.553 2.25 Aromatics vol %60 0 A/F stoich 15.1 Viscosity mm²/s @ 30° C. 0.569 Distillation IBP °C. 98.5 99 T5 ° C. 102.0 99 T10 ° C. 102.0 99 T20 ° C. 102.5 99 T30 ° C.103.0 99 T40 ° C. 103.0 99 T50 ° C. 103.5 99 T60 ° C. 104.0 99 T70 ° C.104.5 99 T80 ° C. 105.0 99 T90 ° C. 106.5 99 T95 ° C. 107.5 99 EP ° C.109.5 99

The effect of higher compression ratio on output torque is shown inFIG. 1. Comparison of the “base” and iso-octane data shows that the peakengine torque is 8% higher at a compression ratio of 13.0 vs. 9.8. Theengine operation for iso-octane is limited to a spark advance of ˜18degrees BTDC due to a knock limitation. Comparison of the DF-2 data tothe iso-octane data shows that not only can the spark advance be setearly enough to reach a plateau in the engine torque output i.e.,operate at MBT but at the same spark advance, there is a significanttorque benefit for fuel DF-2 vs. iso-octane. The combination of highercompression ratio and fuel-derived benefits leads to significantimprovement in overall torque of 11.8%.

FIG. 2 shows the engine brake efficiency vs. spark advance foriso-octane and fuel DF-2. Comparison of the base and iso-octane datashows that the increase in compression ratio from 9.8 to 13.0 enabled byoperating on isooctane raises the relative efficiency by ˜11.6% . Thehigh octane DF-2 allows the engine to be operated at a sufficientlyearly spark advance to reach MBT at 13 CR giving an added benefit overthat for isooctane. The overall benefit associated with using the highoctane fuel DF-2 is an increase in relative brake efficiency of ˜14.6%.

FIG. 3 shows burn curves for both iso-octane and fuel DF-2, from whichit can be seen that fuel DF-2 exhibits a faster heat release rate (rightfigure). This is corroborated by the data in the table at the bottom ofthe figure, which shows that fuel DF-2 takes fewer engine crank anglesto reach both 50% and 90% burn. This faster burn releases more energynear top dead center, resulting in higher efficiency.

The benefits of the high octane fuel DF-2 are identified in thefollowing table. TABLE 2 % Credit % Credit Fuel in Torque in EfficiencyRegular Gas — — Iso-octane  7.8 11.6 DF-2 11.8 14.6

Example 2

The effects of a low octane, low autoignition-resistant, high burn ratefuel on combustion efficiency and performance were investigated the samein-line 4-cylinder (2.0 L displacement) DOHC 4 valve direct injectionspark ignition engine described in Example 1. The engine was operated atvarious low and moderate load conditions at a compression ratio of 9.8and 13.0. The base fuel was a commercial Japanese regular gasoline,named LFG-2B, with a RON value of 91.7. The low octane test fuel, namedDF-1, was comprised of 68% iso-octane, 22% n-heptane, and 10% toluene(measured RON=83.8). The fuel properties are shown in Table 3: TABLE 3Fuel Properties DF-1 Cal- Test Fuel Measured culated RON91 LFG-2BDensity g/cm3 @ 0.7091  0.7094 0.6931 0.7356 15 C. RON — 83.8 80? 9191.7 MON — 82.2  ? 91 82.7 LHV kj/g 43.91 44.5 43.0 H/C mol/mol 2.164 2.112 2.25 1.87 A/F stolch 14.900 15.1 14.7 Viscosity mm2/s @ 0.603 30C. Distillation IBP deg C. 95.0 Approx. 31.5 99 T5 deg C. 98.0 42.5 T10deg C. 98.0 50.5 T20 deg C. 98.5 62.0 T30 deg C. 98.5 72.5 T40 deg C.98.5 85.5 T50 deg C. 98.5 101.0 T60 deg C. 98.5 114.5 T70 deg C. 98.5127.5 T80 deg C. 98.5 144.5 T90 deg C. 98.5 157.0 T95 deg C. 98.5 164.5EP deg C. 120.0 Approx. 175.5 99 Aromatics vol % 10 0 28.7

A comparison of torque output vs. injection timing is shown in FIG. 4for fuel DF-1 and the base fuel LFG-2B at engine conditions of 1200 rpmand fixed spark timing=23 degrees BTDC. Significantly higher torquevalues (left figure) and generally lower torque fluctuations (rightfigure) are realized with fuel DF-1. The DF-1 fuel also generatessignificantly lower NO_(x), HC, and smoke emissions (see FIG. 5). Theeffect of compression ratio on efficiency is shown in FIG. 6, whichshows brake efficiency vs. injection timing for LFG-2B at CR=9.8 (base)and 13.0. The overall boost in relative efficiency realized by higher CRoperation is ˜1.5%. The effect of fuel composition on overall relativeefficiency even larger than this, as is shown in FIG. 6. The relativeefficiency increase associated with combusting DF-1 vs. LFG-2B is ˜5.5%,for an overall relative efficiency gain of 7%. The relative efficiencybenefits are summarized in Table 4. TABLE 4 % Credit in Relative FuelEfficiency LFG-2B (CR = 9.8) — LFG-2B (CR = 13) 1.5 DF-1(CR = 13) 5.5Total 7.0

FIG. 7 shows the burn curves for DF-1 and LFG-2B at identical injectionand spark advance timings of Spark Timing: 23 degrees BTDC, InjectionTiming: 54 degrees BTDE. As can be seen, the burn curve for Fuel DF-1shows two stages of heat release. This heat release behavior isindicative of multipoint autoignition that occurs with the lower octanefuels. Even though the overall average burn rate for these fuels iscomparable, both fuels being relatively high in burn rate, the datashowing higher efficiency and lower emissions demonstrate the importanceof maintaining low RON to get the benefits of autoignition.

Example 3

The effects of a low octane, low autoignition-resistant, high burn ratefuel on combustion efficiency and performance have been investigated ata different region of the driving cycle in the same in-line 4-cylinder(2.0 L displacement) DOHC 4 valve direct injection spark ignition enginedescribed in Examples 1 and 2. The engine was operated at an enginespeed of 3000 rpm and fuel/air ratio of φ=0.56, which is located on adifferent part of the speed/load map than the engine conditionsdescribed in Example 2. The engine was operated at a compression ratioof 9.8 and 13.0. The base fuel was a commercial Japanese regulargasoline, named LFG-2B, with a RON value of 91.7. The low octane testfuel, named DF-1, is the same fuel described in Example 2, and iscomprised of 68% iso-octane, 22% n-heptane, and 10% toluene (measuredRON=83.8). The fuel properties are shown in Table 3: As was observedunder the engine operating conditions of Example 2, significantly lowerNO_(x) and smoke emissions are observed with Fuel DF-1 than with thebase fuel LFG-2B (see FIG. 8).

The effect of compression ratio on relative efficiency is shown in FIG.9, which shows relative brake efficiency vs. injection timing for LFG-2Bat CR=9.8 (base) and 13.0. The overall boost in relative efficiencyrealized by higher CR operation is ˜3%. The effect of fuel compositionon overall relative efficiency is even larger than this, as is shown inFIG. 8. The relative efficiency increase associated with combusting DF-1vs. LFG-2B is ˜5%, for an overall relative efficiency gain of 8%. Therelative efficiency benefits are summarized in Table 5. TABLE 5 % Creditin Relative Fuel Efficiency LFG-2B (CR = 9.8) — LFG-2B (CR = 13) 3DF-1(CR = 13) 5 Total 8

Example 4

The effects of fuel octane and autoignition-resistance on combustionefficiency and performance have been investigated at medium load in thesame in-line 4-cylinder (2.0 L displacement) DOHC 4 valve directinjection spark ignition engine described in Examples 1-3. The enginewas operated at an engine speed of 2400 rpm and fuel/air ratio ofφ=0.63, which is located on a different part of the speed/load map thanthe engine conditions described in Example 2 and 3. The engine wasoperated at a compression ratio of 9.8 and 13.0. Two base fuels wereused in this study; the first was a commercial Japanese regulargasoline, named LFG-2B, with a RON value of 91.7. The second was a blendof 91% iso-octane and 9% n-heptane, named RON91, with a RON value of 91.The low octane test fuel, named DF-1, is the same fuel described inExample 2 and 3, and is comprised of 68% iso-octane, 22% n-heptane, and10% toluene (measured RON=83.8). The fuel properties are shown in Table3. As was observed under the engine operating conditions of Example 2and 3, significantly lower NOx and smoke emissions are observed withFuel DF-1 than with the base fuel LFG-2B (see FIG. 10).

The effect of compression ratio on torque output is shown in FIG. 11,which shows relative torque output vs injection timing for LFG-2B atCR=9.8 and 13.0. Also shown are data for RON91 and DF-1. Unlike the twoprevious examples, the low octane fuel DF-1 has lower relative torqueoutput than the higher octane fuels. Similarly, the engine relativeefficiency is lower with the low octane fuel DF-1 than with RON91 andLFG-2B (see FIG. 12). The reason for the diminished performance is thatthe engine cannot operate with the low octane fuel DF-1 with sparkadvance timings that approach MBT due to knock limitations. These datademonstrate that at intermediate loads, fuel properties (octane levelsand composition) more commensurate with conventional gasoline are moresuitable than the low octane fuels (such as fuel DF-1).

Example 5

The effects of a low octane, low autoignition-resistant, high burn ratefuel on combustion efficiency and performance have been investigated inan in-line 4-cylinder (2.0 L displacement) DOHC 4 valve direct injectionspark ignition engine similar to the engine described in Examples 1-4.Then engine had a swirl injector rather than the fan spray injectordescribed in Examples 1-4 and was operated at a lower compression ratioof 10.3. The engine was operated at an engine speed of 1200 rpm andfuel/air ratio of φ=0.5. The base fuel was 100% iso-octane (RON=100) andseveral low octane test fuels were studied, i.e., n-hexane (RON=25),2-methylpentane (RON=69), and cyclohexane (RON=84).

Burn curves for these fuels are shown in FIG. 13. Several observationsare noteworthy. First, the burn curve for n-hexane is the most rapid andreaches 80% burn completion much quicker than the other fuels. By virtueof this, the overall efficiency is 8% higher than iso-octane. Second,the NO_(x) levels for n-hexane are much lower than iso-octane. Thisreflects the very fast heat release, and the tendency to form lessNO_(x) when the combination of high temperature and time is minimized.Third, relative efficiency benefits similar to those identified forn-hexane are observed with the other two low octane fuels, i.e.,2-methylpentane and cyclohexane, where credits of 2% and 6% areobserved, respectively. The high relative efficiency of these low octanefuels reflects the fast burn rates of the low octane fuels. This highburn rate has two primary contributing factors, i) high laminar flamespeed, and ii) controlled autoignition. High laminar flame speed is theprimary factor responsible for the high relative efficiency ofcyclohexane, while autoignition is likely to be the main factorresponsible for the increased relative efficiency of n-hexane and2-methylpentane. This is evident in FIG. 14, which shows heat releasecurves for these fuels. The very rapid heat release for n-hexane ispostulated to originate from multipoint autoignition initiated by endgas compression from the flame front and piston movement. It is worthnoting that under these conditions, autoignition does not generate theheat release levels typically encountered under knocking conditions athigher load, and thus no deleterious effects associated withautoignition are observed.

It is important to note that while these data were obtained in an enginewith a compression ratio of 10:1, the benefits of low octane areexpected to be realized at higher CR as well. This was demonstrated inExamples 2 and 3, where increasing the CR from 9.8 to 13 led to higherefficiency at all loads and speeds. The further efficiency and emissionbenefits observed for these examples with the low octane fuel are alsoexpected to realized with these fuels in a higher CR engine undersimilar operating conditions.

1-12. (canceled)
 13. A fuel system for spark ignition engines having aCR of 11 or more, comprising: at least a first fuel and a second fuel;the first fuel having a RON greater than 100, a burn rate greater than105% of isooctane and a laminar flame speed greater than 105% ofisooctane; means for injecting the first fuel into the engine forcombustion therein in response to at least a first predetermined engineoperating condition; a second fuel having a RON less than 90, a burnrate greater than 105% of isooctane and a laminar flame speed greater105% of isooctane; and means for injecting the second fuel into theengine for combustion therein in response to at least a secondpredetermined engine operating condition.
 14. The fuel system of claim13 including at least a third fuel having a RON between that of thefirst and second fuel, further characterized as having a burn rate andflame speed greater than 105% of isooctane.
 15. The fuel system of claim14 wherein the third fuel is admixed from the first and second fuel. 16.The fuel system of claim 15 wherein the admixture functions to allowengine operation at or about MBT.
 17. The fuel system of claim 14wherein the third fuel functions to allow engine operation at or aboutMBT.
 18. A method for operating spark ignition, internal combustionengine having a CR of 11 or more comprising: combusting a first fuelhaving a RON greater than 100,a burn rate greater than 105% of isooctaneand a laminar flame speed greater than 105% of isooctane under high loadconditions; and combusting a second fuel having a RON less than 90, aburn rate of greater than 105% isooctane and a laminar flame speedgreater than 105% isooctane under low load conditions.
 19. The method ofclaim 18 having the additional step of combusting a third fuel undermoderate load conditions wherein said third fuel has a RON between thatof the first and second fuel and further characterized as having a burnrate and flame speed greater than 105% of isooctane.
 20. The method ofclaim 19 wherein the third fuel is admixed from the first and secondfuel.
 21. The method of claim 20 wherein the admixture functions toallow engine operation at or about MBT.
 22. The method of claim 19wherein the third fuel functions to allow engine operation at or aboutMBT.
 23. A method for operating a vehicle having a spark ignition engineto increase the efficiency and reduce the emissions of the engine underconditions of use comprising: supplying a first fuel to the engine atabout high engine load conditions; and supplying a second fuel to theengine at about low engine load conditions, the first fuel having a RONgreater than 100, a burn rate greater than 105% of isooctane and alaminar flame speed greater than 105% of isooctane; the second fuelhaving a RON less than 90, a burn rate greater than 105% of isooctaneand a laminar flame speed greater than 105% of isooctane; and wherebyengine efficiency is increased and emissions are reduced.
 24. The methodof claim 23 wherein said engine is a direct injection, stratified chargeengine.
 25. The method of claim 23 wherein said engine is a port fuelinjected, stratified charge engine.
 26. The method of claim 23 having anadditional step of supplying a third fuel to the engine at aboutmoderate load levels, said third fuel having a RON less than about 100and greater than about
 90. 27. The method of claim 26 wherein the thirdfuel is admixed from the first and second fuel.
 28. The method of claim27 wherein the admixture functions to allow engine operation at or aboutMBT.
 29. The method of claim 26 wherein the third fuel functions toallow engine operation at or about MBT.
 30. A fuel system for sparkignited engines that operate under high exhaust gas recycle during lowto moderate engine load conditions the system comprising: means forsupplying to the engine a first fuel during high load conditions; andmeans for supplying to the engine a second fuel during low loadconditions; the first fuel having a RON greater than 100, a burn rategreater than 105% of isooctane and a laminar flame speed greater than105% of isooctane; the second fuel having a RON less than 90, a burnrate greater than 105% of isooctane and a laminar flame speed greaterthan 105% of isooctane; and whereby engine efficiency is increased andemissions are reduced.
 31. The method of claim 30 further includingmeans for supplying to the engine a third fuel during moderate loadconditions, said fuel having a RON greater than about 90 and less thanabout 100, a burn rate and flame speed greater than about 105% ofisooctane.
 32. The method of claim 31 wherein the third fuel is admixedfrom the first and second fuel.
 33. The method of claim 32 wherein theadmixture functions to allow engine operation at or about MBT.
 34. Themethod of claim 31 wherein the third fuel functions to allow engineoperation at or about MBT.
 35. In a vehicle having spark ignitionengine, the improvement wherein the engine has a CR of 11 or more;wherein at least a first fuel and a second fuel are available on thevehicle for combustion by the engine, the first fuel having a RONgreater than 100, and under high load conditions a burn rate greaterthan 105% of isooctane and a laminar flame speed greater than 105% ofisooctane, and the second fuel having a RON less than 90, and under lowload conditions a burn rate greater than 105% of isooctane and a laminarflame speed greater than 105% of isooctane; and wherein the first fuelis supplied to the engine when operating under high load conditions andthe second fuel is supplied to the engine when operating under low loadconditions.
 36. The vehicle of claim 35 wherein a third fuel isavailable on the vehicle and is supplied to the engine thereof, saidfuel having a RON greater than about 90 and less than about 100, and aburn rate and flame speed greater than about 105% of isooctane.
 37. Thevehicle of claim 36 wherein the third fuel is admixed from the first andsecond fuel.
 38. The vehicle of claim 37 wherein the admixture functionsto allow engine operation at or about MBT.
 39. The vehicle of claim 36wherein the third fuel functions to allow engine operation at or aboutMBT.
 40. A method of operating an internal combustion engine having a CRof 11 or more, the method comprising: providing a plurality of fuels ofdifferent and predetermined combustion properties, each fuel selected toimprove engine performance under preselected operating conditions; andsupplying the selected fuel to the engine when operating at thepreselected condition.
 41. A method of reducing emissions and increasingefficiency of a spark ignition internal combustion engine having a CR of11 or more, the method comprising: providing a plurality of fuels ofdifferent and predetermined combustion properties, each fuel selected toimprove engine performance under preselected operating conditions; andsupplying the selected fuel to the engine when operating at thepreselected condition.
 42. A fuel system for spark ignition engineshaving a CR of 11 or more, comprising: at least a first fuel and asecond fuel; the first fuel having a RON set at the minimum required toallow operating the engine at MBT when at wide open throttle at the rpmsetting for maximum power, a burn rate greater than 105% of isooctaneand a laminar flame speed greater than 105% of isooctane; means forinjecting the first fuel into the port or engine for combustion thereinin response to at least a first predetermined engine operatingcondition; a second fuel having a RON less than 90, a burn rate greaterthan 105% of isooctane and a laminar flame speed greater than 105% ofisooctane; and means for injecting the second fuel into the port orengine for combustion therein in response to at least a secondpredetermined engine operating condition.
 43. The fuel system of claim42 further comprising a third fuel having a RON greater than about 90and less than about 100, and a burn rate and flame speed greater thanabout 105% of isooctane.
 44. The fuel system of claim 43 wherein thethird fuel is admixed from the first and second fuel.
 45. The fuelsystem of claim 44 wherein the admixture functions to allow engineoperation at or about MBT.
 46. The fuel system of claim 43 wherein thethird fuel functions to allow engine operation at or about MBT.